1. Field of the Invention
The present invention relates to a self acting gas bearing apparatus equipped with dynamic pressure generating grooves which is used in spindle sections of polygon mirror scanners, hard disk drive units and other devices.
2. Description of the Prior Art
Among the self acting gas bearing apparatuses of various constitutions proposed so far, an apparatus as shown in FIG. 5, has been known to be applied to a polygon mirror scanner. The conventional dynamic pressure gas bearing apparatus comprises a bearing body 22 with a shaft 23 fixed at the center thereof. Fitted to the shaft 23 is a sleeve 24 that is freely rotatable with a proper clearance therebetween. Formed on the outer surface of the shaft 23 are dynamic pressure generating grooves 23a and 23b of a herring-bone-like configuration and dynamic pressure generating grooves 23c of a helical configuration. Fixed on the bearing body 22 is a stator 25 of a motor 27, and fixed on the sleeve 24 is a rotor 26. Fixed on the top of the sleeve 24 is a thrust bearing member 28 which has a throttle hole 28a. Fixed on a flange section 24a of the sleeve 24 is a polygon mirror 29 via a set plate 30 by means of screws. A dust cover 31 and a polished glass 32 are provided for the purpose of preventing the entry of dust into the bearing clearance.
The self acting gas bearing apparatus of a constitution as described above, works as follows: First, when the motor 27 is energized and the rotor 26 starts to rotate, the sleeve 24 rotates without coming into contact with the shaft 23 by a pumping action of the dynamic pressure generating grooves 23a and 23b. When the motor 27 is at a stand-still, the shaft 23 and the thrust bearing member 28 are in contact with each other. However, once the sleeve 24 rotates, the pumping action of the helical-shaped dynamic pressure generating grooves 23c causes the gas around the bearing to be drawn in the direction indicated by arrows B and discharged through the throttle hole 28a in the direction indicated by arrow C, thereby generating a lifting force in the direction indicated by arrow A to lift the sleeve 24. Laser beam from a light source (not shown) outside of this self acting gas bearing transmits through the polished glass 32 in the direction indicated by arrow D and is reflected by the polygon mirror 29.
In such a constitution, however, while the gas around the bearing is drawn from the clearance between the sleeve 24 and the bearing body 22 in the direction indicated by the arrow B by the helical-shaped dynamic pressure generating grooves 23c of the shaft 23 during a rotation of the sleeve 24, fine particles of several micrometers from the surroundings, which are, for example, generated from the coil insulation film of the stator 25, enter the bearing clearance between the shaft 23 and the sleeve 24, eventually causing seizure of the bearing. The dust cover 31 which covers the entire surface of the unit and the polished glass 32 are needed, which makes the production cost high.